The increasing demand for high purity chemicals requires development of hyperselective separation processes. Compared to distillation, separation by adsorption on solid surfaces allows achieving much higher selectivities (Ruthven, 1984; John Wiley and Sons: New York).
Among the broad range of industrial adsorbents, zeolites represent a particular family. Whereas with conventional materials, discrimination results from specific energetic interactions between molecules and the adsorbent surface, the selectivity of zeolites arises from the subtle matching of size and shape of guest molecules and zeolite micropores (Chen et al, 1989; Marcel Dekker, Inc.: New York).
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities, which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of ways to take advantage of these properties.
Various types of zeolites can be used for separating various hydrocarbon types such as aromatics, alkanes or alkenes, from each other. Specific zeolites can be utilized which have been base exchanged to allow the various cations to be utilized advantageously in performing a predetermined selective adsorption of a hydrocarbon type.
In common industrial adsorptive separation processes using zeolites, straight chain alkanes are adsorbed into the pores of the zeolites, while the more branched alkanes are excluded from entering the pores. Adsorption of branched molecules is essentially restricted by the small pore size of the zeolites. Hence, mono- and multi-branched alkanes are not separated from each other. Therefore, there remains a need in the art for providing a method for separating the more bulky, mono-branched molecules from the multi-branched and the linear molecules.
U.S. Pat. No. 6,069,289 describes a process to separate multimethyl-branched alkanes from a mixture of multimethyl-branched alkanes, monomethyl-branched alkanes and normal alkanes. The adsorbent particles in this process have a selectivity order from normal alkanes to monomethyl-branched alkanes further to multimethyl-branched alkanes.
US 20002/0045793 discloses a process for separating multi-branched alkanes comprised in a hydrocarbon feed using at least one zeolitic adsorbent. The adsorbent preferably adsorbs linear alkanes, to lesser extent mono-branched alkanes and finally only minor amounts of multi-branched alkanes, naphthenic compounds and aromatic compounds.
U.S. Pat. No. 5,107,052 describes the selective adsorption of multi-branched alkanes on SAPO-5, AlPO4-5, SSZ-24, MgAPO-5 or MAPSO-5 zeolites. These zeolites are aluminophosphates, except SSZ-24 which is a all-silica zeolite that is isostructural with AlPO4-5, and characterized in that they have pores large enough to admit all components of a mixture of linear, mono- and di-branched alkanes, and selectively adsorb di-methyl alkanes but no mono-methyl and normal alkanes.
The present invention aims to provide an improved separation method, which favors adsorption of the mono-branched hydrocarbons, to lesser extent linear hydrocarbons and finally only minor amounts of di- and multi-branched hydrocarbons. Even more in particular, it is an object of the present invention to provide an improved method that favors adsorption of mono-branched alkanes, to lesser extent linear alkanes and finally only minor amounts of di- and multi-branched alkanes.
Another object of the present invention is to provide an improved method for separating mixtures of hydrocarbons into fractions enriched in linear, mono-branched or multi-branched hydrocarbons. Even more in particular, it is the object of the present invention to provide an improved method for separating mixtures of mono-branched alkanes into fractions enriched in linear, mono-branched or multi-branched hydrocarbons.
The present invention has the object to provide such separation methods using zeolite-based adsorbents to preferentially adsorb mono-branched hydrocarbons.